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Black carbon particles are composed of pure carbon, linked via several types of bonds. The particles are formed by the incomplete combustion of fossil fuels, biofuels, and biomass. When they're suspended in the atmosphere, these particles can mix with other aerosols, including organics and sulfates, and they can directly absorb sunlight. So once in the air, black carbon can both influence the climate and impact air quality.

Once suspended in the atmosphere, black carbon's warming effect can last days to weeks; it also absorbs a broad range of light. However, it is still unclear whether this absorption changes as the particles undergo chemical reactions in the atmosphere—changes that could have big implications for its influence on the climate.

Now, an international team of scientists developed an environmental chamber that allowed them to study the evolution of black carbon's properties in the atmosphere. They tested their new technique on pollution from cities in the US and China.

Direct measurements of black carbon aging under ambient conditions can be difficult to interpret, as they're affected by many factors—transport of particles, local emissions, and the local atmospheric chemistry. These factors need to be decoupled to quantify the evolution of black carbon particles.

To handle this, the team developed a novel environmental chamber that allowed them to track the aging of and variation in black carbon particle properties under ambient conditions without the interference of ambient aerosols. The team conducted experiments in Houston, USA (from May to June 2009) and Beijing, China (from August to October 2013).

Their work revealed that black carbon ages through two distinct stages. First, there is an initial transformation from a fractal to a spherical shape. During this stage, there is little variation in absorption of light (and other electromagnetic radiation). Next, they saw the growth of fully compact particles that exhibit a significant enhancement in light absorption—the ability of black carbon to warm the earth goes up at this point.

But, this evolution did not occur at the same rate in both cities. In Beijing, it took 2.3 hours for the particles to change shape, and the absorption increased by a factor of 2.4 in a bit under five hours. In Houston, it took longer to achieve the same conditions—nine hours and 18 hours, respectively.

The scientists think that the black carbon grows more quickly in Beijing because of the higher pollution levels. One indication that the pollution was higher in Beijing was the higher level of gaseous aerosols present. During their measurements, the ambient xylene and toluene concentrations (two organic solvents) exceeded 10 ppb in China, which was significantly higher than in Houston. They also found that the contributions of organic and inorganic (sulfate and nitrate) species to the black carbon mass fluctuated in Beijing.

Another indication of the increased pollution in Beijing was the presence of visual haze in the atmosphere caused by suspended dust, smoke, and small particles. A chemical analysis of the black carbon revealed a large coating of organics formed on black carbon particles during the initial stage of haze development in Beijing. Small coatings (<0.5 nm) had little effect on the particles' absorption of light. The maximum absorption enhancement factor they measured in Beijing was 2.4, which is actually larger than those used in commonly used global climate models.

This study revealed that the ability of black carbon to warm the atmosphere via light absorption is influenced by particle aging; it also indicated that pollution levels strongly influence the rate of black carbon aging. And the aging had an effect on visibility, air quality, and weather—all through the enhanced light absorption of black carbon.

As a result, reduction of black carbon emission could both help control air pollution and moderate climate change.